The wettability of the rock in an oil reservoir affects the saturations, capillary pressures (Pc), electrical properties, relative permeability and ultimate hydrocarbon recovery. In the petroleum industry, semi-empirical indices based on Pc curve measurements are used to describe core plug wettability: (1) Amott or Amott-Harvey index IAH, and (2) U.S. Bureau of Mines (USBM) index, IUSBM. Frequently those indices do not match, despite industry expectation.
NMR is a technique that has been shown to be very sensitive to rock/fluid interfaces. The most widely used application is the measurement of the pore size distribution when the pores are all 100% saturated with a single phase such as water. In such cases, the interpretation is straightforward and is based on the fact that as the pore size decreases, the surface to volume ratio increases resulting in shorter (i.e., enhanced) T2 relaxation times. It is also well known that the NMR T2 relaxation times are highly sensitive to the presence of fractionally saturated (oil and water) and mixed wet pores, such as in the case of core-plugs in their native state taken from an oil zone.